Current concepts of cellular immunity suggest that interaction of lymphocytes and macrophages is important in host defense against intracellular bacterial pathogens such as Mycobacterium leprae. Despite mounting evidence of abnormal function of thymus-mediated lymphocyte function in lepromatous leprosy, it remains unclear whether this is the cause or the result of unrestricted multiplication of the leprosy bacillus. Thus, there has been considerable speculation, and some evidence, that it is the macrophage which is basically defective in those patients destined to have the lepromatous form of the disease. The proposed studies will examine the relative capabilities of lymphocytes and macrophages from tuberculoid and lepromatous leprosy patients to interact in tests of both the afferent and efferent limbs of cellular immunity. Thus, the influence of macrophages on the blastic transformation of lymphocytes, and of lymphocytes on the activation, migration, and bactericidal and digestive capabilities of macrophages will be explored. Evidence will be sought relative to a specific bactericidal or digestive ("processing") defect for M. leprae in cells of patients with lepromatous leprosy.